In situ combustion method



CONDENSATION ZONE DISTANCE w c. HARDY L IN SITU COMBUSTION METHOD FiledDec. 15-, 1967 CONDENSATION ZONE DISTANCE ww In mDOmDG mIP 2.

July 1, 1969 COMBUSTION ZONE TEMPERATURE TEMPERATURE AMBIENT ZONETEMPERATURE COMBUSTION ZONE FIG. 3

United States Patent US. Cl. 166256 7 Claims ABSTRACT OF THE DISCLOSUREThe particular embodiment described herein as illustrative of one formof the invention utilizes'a method for applying in situ combustiontechniques to formations having a high gravity crude oil, which methodinvolves the generation of a combustible fuel in the formation by theaddition of acids to the formation. The mixture of acid and crude oilproduces an acid sludge, with the order of the reaction being increasedby elevated temperature. The combustion of such an acid sludged fuelwill liberate water and acid gases. These products will flow as amixture to cooler regions of the reservoir where condensation andabsorption takes place to form a regenerative cycle of acid.

Background of the invention This invention relates to a method forrecovery of hydrocarbons from oil bearing formations, and has particularreference to recovery processes involving in situ combustion of highgravity crude oil.

Various methods of applying heat to an oil reservoir are known, however,the forward burn technique is the most common and the process set forthin this-disclosure will be described in terms of the forward burn. Theforward burn consists principally of (1) flowing air through an oilreservoir from an injection well to a producing well, (2) igniting theoil around the injection well by some convenient method such aselectrical heater, gas burner or chemical reaction, and (3) continuingthe injection of air to propagate a combustion zone through the oilreservoir from injection to producing wells. Low gravity crude oils,particularly those ranging between and API, appear to afford littleditficulty in the in situ combustion process in that a properaccumulation of fuel is maintained for the encroaching combustion front.High gravity crude oils, on the other hand, generally have lowviscosities and low boiling ranges and therefore tend to be displaced bythe heat of the combustion taking place in the formation therebypresenting an insufficient accumulation of fuel to the encroachingcombustion front. These oils, ranging, for example, from to API,generally do not naturally contain sufficient heavy fractions to fuelthe forward combustion process. Although this type of oil generallywater floods fairly well, frequently large volumes remain in thereservoir as residual.

It is therefore an object of the present invention to provide a new andimproved method of in situ combustion in high gravity crude reservoirsby thickening a small fraction of the oil to convert it to fuel forsupporting the combustion necessary to produce the balance ofhydrocarbons in the reservoir.

Summary of the invention With these and other objects in view, thepresent invention contemplates a method for recovering hydrocarbons froma formation by in situ combustion where the formation contains highgravity crudes, including adding or generating an acid in the formationwhich when contacted with the high gravity crudes forms a petroleumsludge. Such sludge acts as a fuel to support the in situ combustionprocess. Air and heat are applied to the formation to ignite the sludge.Acid gases and water vapor are liberated from the combustion process andmigrate with the air flow to cooler regions of the formation wherecondensation of the acid gas and water produces an acid which, in turn,forms a regenerative cycle of acid sludge. The initial introduction ofacid to the formation may be in the form of an acid, an acid forminggas, a petroleum product containing compounds capable of generating anacid or acid gas, or any other material which will produce an acid whencombined with materials in a petroleum formation under the conditions ofan in situ combustion process.

A complete understanding of this invention may be had by reference tothe following detailed description when read in conjunction with theaccompanying drawings.

A brief description of the drawing FIGURE 1 is a diagram, in the form ofa vertical section illustrating a portion of an oil reservoir to whichthe invention is applied;

FIGURE 2 is a plot of temperature versus distance to the horizontalscale of FIGURE 1, explanatory of the type of problem which exists andwhich it is the object of the present invention to solve; and

FIGURE 3 is a plot similar to FIGURE 2 illustrating the variation ofhydrogen ion concentration in the aqueous phase of a well versusdistance.

Description of the preferred embodiments Referring first to FIGURE 1,one or more injection Wells are indicated at 12, provided with controlvalves 14, and penetrating at their lower end 16, a hydrocarboncontaining formation indicated at 18. Spaced from the injection wellsare production wells 20 illustrated as having control valves 22, whichwells penetrate the same formation at 24. It will be understood thatwhile these wells are merely diagramed, they may be arranged and spacedin accordance with well known practices which need not be described indetail here since they have no bearing on the essential aspects of thepresent invention. The production wells may, of course, be pumped if required. The wells are also located relative to one another in accordancewith conventional practices depending upon knowledge of the producingformation, this knowledge being secured by preliminary conventionalproduction, by the drilling of test holes, by core sampling, et cetera.In brief, the arrangement of wells is adapted to secure, ultimately,maximum recovery with minimum cost. It might be stated, as a summary,that the locations are such as to secure a sweeping of desired productsfrom a maximum region consistent with the above burning procedure.

Considering the wells indicated in FIGURE 1, a formation between theinjection well or wells and the production well or wells, in which insitu combustion is progressing, contains identifiable zones, though theboundary of transition from one zone to the next may not be sharplydefined; in fact, they may be quite indefinite. From the standpoint ofconsideration of the present invention, the zones which would exist inthe ordi nary forward combustion procedure, after burning had beeninitiated, to some extent would be as generally illustrated in FIGURE 2.First, in the vicinity of the injection well or wells there would 'be aburned zone indicated at 26. Following this, in the direction of flow,there is a combustion zone indicated at 28, a condensation zoneindicated at 32, and finally the unaltered zone 34. The burned zonecontains approximately of the range in temperature up to 1000 degrees F.The combustion zone may range from 550 degrees F. to 1200 degrees F.,depending upon the type and amount of fuel deposited by the oil on thereservoir rock. The condensation zone which actually controls the entireprocess may be viewed as three subzones; subzone 36 containing highviscosity distillation residue, subzone 38 containing a high watersaturation and residual oil, and subzone 40 containing the oil bank. Itis apparent, that in order to propagate a forward in situ combustionprocess, there must be an accumulation of hydrocarbon in subzone 36which will furnish fuel for the encroaching combustion front. Highgravity crude oils, which generally have low viscosities and low boilingranges tend to be readily displaced from this zone thereby causing aninsuificient accumulation of oil for fuel in subzone 36.

Crude oil in which the ordinary combustion process is sufiicientlyoperative has been found to exhibit particular physical properties.Those properties which have been found to thus distinguish between crudeoils which are or are not susceptible to forward combustion processesare the average volumetric boiling point (A.V.B.P.) and the viscosity at350 degrees F. Crude oils whose average volumetric boiling point(A.V.B.P.) viscosity (350 F.) product exceeds 350 F. C.P. will generallypropagate in situ combustion process. Crude oils whose (A.V.B.P.)viscosity product is less than 350 F. C.P. have been found incapable ofor very inefficient in propagating a forward in situ process.

It is well known that the combustion of crude oil in air will give riseto the production of water, combustion gases such as CO and CO oxides ofsulphur, oxides of nitrogen, oxygenated hydrocarbon, plus organic andinorganic acids. In the forward in situ combustion process, theseproducts flow from the combustion zone to cooler regions of theformation where they are condensed and/or sorbed from the vapor phase.The presence of the oxides of carbon, sulphur, and nitrogen in additionto water causes the formation of inorganic acids such as carbonic acid,sulphuric acid and nitric acid downstream from the combustion zone.Analysis has shown that in addition to inorganic acids, organic acidssuch as formic, acetic, propionic, butyric, valeric, caproic and othersup to ten carbon molecules are also found in condensed products ofcombustion. In addition to fatty acids, the combustion products havebeen shown to contain hydroxy acids, lactones, anhydrides, phenols,naphthenic acids, alcohols, .aldehydes, ketones, and esters.

That the presence of organic and inorganic acids in the products ofcombustion are formed by condensation or sorption of the combustionproducts can be indicated by the hydrogen ion concentration or pH of theproduced water from the in situ combustion process. FIGURE 3 shows atypical curve of pH versus distance, of the water contained in theformation which hosts in the in situ combustion process. FIGURE 3 showsthat the pH of the water in the ambient zone 34 should be very near thatof the original water in place. This value will vary, depending upon thesalts in solution; however, the original water in place is generallyslightly acid. The water in the condensation zone 32 of the in situcombustion process has been found to have an average pH of approximatelythree. This value corresponds generally to the pH of carbonic acidresulting from the solution of CO in water. As the combustion zone 28 isapproached, it has been found that the pH of the water in the in situcombustion process drops to a value as low as 1.0 or lower. Thisproperty of the water indicates a concentration of fairly strong organicand/or inorganic acids.

In the processing of crude petroleum oils, especially in refining, it isgenerally known that the mixing of oil with various organic andinorganic acids will produce a sludge which will precipitate from theoil. Especially well known is the de-asphalting process which consistsof the admixture of concentrated sulphuric acid and crude oils. Also,the sludging of even refined lubricating oils can be observed in thecrankcases of internal combustion engines, due to its contamination withacids manufactured by the combustion of gasoline in the enginescylinders. It is also known that in certain formations, followingacidizing treatments, the Wells have been very slow to clean up, andoften .a great deal of asphalt like material has been recovered with thetreating fluids. In some cases, complete or partial plugging of thewells has resulted from the treatment. The study of the problem hasrevealed that the crude oils produced from such wells actually for-msolid precipitates upon contact with the acid. These precipitates weremainly asphaltenes, resins, paraffin waxes, and other high molecularweight hydrocarbons. Experiments have shown that the sludges formed fromcrude oil by .acid treatment are generally sticky and characterized byhigh viscosity, high molecular weight, and relatively low ignitiontemperatures.

It has been found by experiment that fuel consumed in the in situcombustion process is, in part, manufactured in situ by the sludging ofcertain components of the crude oil when reacted with the complex acidsresulting from combustion. The mechanism of sludging a crude oil withacid is not entirely understood, however, others have shown (PetroleumRefinery Engineering by W. L. Nelson, McGraw-Hill, 1949, chapter 12)that sulphuric and nitric acids will react with oil to remove nitrogen,sulphur, and oxygen based compounds. These acids will react readily withunsaturated hydrocarbons. In their order of reactivity, the followingcompounds commonly found in crude oil will produce acid sludges: (1)asphaltic compounds, (2) olephenic compounds, (3) aromatic compounds,and (4) naphthenic compounds. Parafiins have been found to react veryslowly with most acids to 'produce acid sludges. The mechanisms whichproduce acid sludges from the admixture of various acids and crude oilare, amongst others: (1) the decrease in solubility of polar,heterohydrocarbon compounds containing nitrogen, sulphur, and oxygen.Asphaltenes .are composed of such compounds. For example, theprecipitation of asphaltenes in high pH environments appears to berelated to decreased solubility of polar nitrogen containing compounds;(2) reaction of acids with unsaturated hydrocarbons to produce polymers,aldehydes, organic acids, and other oxidation products; (3) dissolutionof naphthenic acids and basic nitrogen compounds; (4) sulfonation andnitration of hydrocarbons to produce, for example, alkyl, aryl, andnaphthenic acids, sulfonates, nitrates, acid esters, etc. The order ofthese reactions is increased by elevated temperature.

Combustion of such petroleum acids has been shown to produce combustionproducts such as CO S0 S0 N 0, NO, N0 and other acid gases. These gaseswhen contacting water can be absorbed to form nitric, sulphuric, andcarbolic acids. Crude oils that do not deposit suflicient fuel on thereservoir rock to sustain the forward in situ combustion process alsoproduce only trace amounts of organic and inorganic acid gases becauseinsuflicient burning takes place, thus leaving a high percentage ofoxygen and a low percentage of combustion gas. On the other hand, crudeoils which do deposit sufiicient fuel on the reservoir rock to sustainthe forward in situ combustion process, also produce a significantamount of acid gases and organic acids. Crude oils which have high insitu combustibility tend to be naphthenic and crude oils which have lowin situ com bustibility tend to be parafiinic. It is generally knownthat parafiinic crude oils will produce relative low volumes of acidsludge.

In order to determine the applicability of acid sludging and subsequentin situ combustion of a reservoir containing high grade crude oils, acombustion tube test was conducted to simulate such .a process. Thecombustion tube test utilized a 42 inch long, thin wall, 5 inch outsidediameter stainless steel tube packed with a mixture of graded sand,clay, water, and crude oil mixed in the desired weight ratio. The packedtube was assembled with an electrical ignitor adjacent the sand face atone end of the tube. The assembled combustion tube was inserted in areactor chamber to permit its operation at high pressures withoutcreating a pressure differential across the walls of the tube. For thetest, 26 cylindrical guard heaters each approximately one and one-halfinches wide were placed about the combustion tube to provide foroperation at near adiabatic conditions. Provision is made for separationof produced liquids .and gases. The combustion tube test is initiated byestablishing air flow at a controlled rate of injection pressure throughthe tube. Ignition of the oil sand is effected by energizing theelectrical ignitor and controlling its temperature to a preset value.The combustion front is propagated through the tube by continuousinjection of air. The efiiuent gas is analyzed and its volume ismeasured. A thermocouple well contains twenty-six thermocouple junctionsarranged to sense the temperature 1 /2 inch intervals along the lengthof the tube. This thermowell is placed in the center of the combustiontube before packing the tube.

After ignition of the oil sand, the high temperature combustion zonemoves down the combustion tube. Temperature changes along the tube aresensed by the thermocouples in the center thermowell and compared withthe temperature of the guard heater to thereby control the guard heatersystem and maintain the individual heaters at the same temperature asthat indicated by the corresponding thermocouple in the combinationtube. The temperature is sequentially measured along the tube and thisparameter is recorded automatically to provide data for temperatureprofiles of the combustion tube at any time. Pressure measurements andprofiles are also made on the test. The eflluent gas is continuouslyanalyzed by an oxygen analyzer and a series of thermal conductivity cellanalyzers. Chromatograph analysis is also run on the efliuent gas todetermine the contents of the combustion gas.

Such a combustion tube test, together with bench tests shows thatcharacteristic behavior of crude oils and components of crude oils whenadmixed with certain acids can be used to cause the deposition ofsuflicient fuel in .a petroleum reservoir to sustain the forward in situcombustion process. Such characteristic behavior may be the action ofsulphuric acid with the aromatic, olefinic, naphthenic, and asphaltcomponents of crude oils to precipitate a sludge containing asphaltenesand various reaction products such as alkyl, aryl, and naphthenicsulfonates, acids, aldehydes, alcohols, and esters.

The .above described behaviors may be applied to petroleum reservoirs,using several techniques, to increase oil recovery by the forward insitu combustion process. One such technique is described as follows: Aflow of air through the strata of a hydrocarbon bearing reservoir isestablished by injecting air through one or more injection wells andreturning a portion of the air to the surface through one or moreproducing wells. A slug of acid is then introduced into the injectionwell which, together with a continued air flow, causes the flow of theacid into the petroleum reservoir. The intimate mingling of the acid andcrude oil within the interstices of porous media will cause a sludge toprecipitate from the oil and stick to the walls of the porous media.Heat is then applied to the reservoir by any well known means to ignitethe sludge formed therein and thereby initiate combustion of thereservoir. The acid injected could be one or a combination of organic orinorganic types of acid. Tests have shown that commercial gradeconcentrated sulphuric acid, red fuming nitric acid, white fuming nitricacid, certain organic acids, halogenated organic acids, and combinationsof such acids will produce satisfactory sludging of crude oil. The typeof acid used would depend to a great extent on the type of crude oilinvolved, with some crudes being more effectively sludged by certaintypes of acid. The specific type of acid needed in a forward in situcombustion process can easily be determined by simple laboratory testssimulating the reservoir conditions.

The volume of the acid slug which would be injected into a reservoir tocause sludging of the crude oil has been found in laboratory experimentsto range between .5 and 5 percent of the recoverable oil in thereservoir.

However, all classes of crudes and acids have not been combined todetermine the optimum percent by volume which would be needed for thevarious types, and therefore these percentages should not be consideredas limiting factors, but rather as indicative of a range which may beapplicable to individual situations. Since the formation of thepetroleum sludge is an acid reaction process, the combustion of thesludge will liberate water and acid gases such as CO S0 S0 N 0, NO, andN0 upon combustion in the presence of oxygen. These products will, inturn, flow as a mixture to cooler regions of the reservoir, that is thecondensation zones where condensation and sorption take place to createa regenerative cycle of acid production. The regenerated acidimmediately reacts with new oil in the reservoir to produce a petroleumsludge on which the forward in situ combustion process is sustained.

An alternative process which utilizes the same concepts as the processdescribed above, is as follows: in conjunction with the steps ofinjecting air into the reservoir, igniting hydrocarbons in the reservoirand producing the reservoir from a production well; where high gravitycrude oils are found, a fuel gas is injected into the reservoirsimultaneously with the injection of air which when reacted with air bycombustion will produce an acid gas. An example of such a gas ishydrogen sulphide, which will react with oxygen to produce S0 and S0 Thefuel gas can be burned either in the injection well or in the formationduring and subsequent to ignition. The acid gas resulting from thecombustion of the fuel gas will flow to cooler regions of the formationwhere it condenses and combines with water in the formation to form anacid which, in turn, establishes the regenerative cycle of acid sludgedescribed previously.

A third technique involving principles of the present invention is alsoapplied in conjunction with the steps of flowing air into a formation,igniting the formation, and recovering production at a recovery well.This technique includes injecting a slug of crude oil into the wellprior to the application of heat for ignition. The crude oil should havea relatively high percentage of oxygen, sulphur, and/or nitrogencompounds. The presence of such compounds under combustion conditionswill cause a natural production of an acid fuel sludge to support aforward in situ combustion process. Again, combustion of such fuelsludge will produce acid gases and water which, upon flowing to coolregions of the formation will condense to form acid and consequently, aregenerative cycle of acid sludge as previously described.

In summary, the processes described above permit the propagation of aforward in situ combustion process in reservoirs containing high gravitycrude oil. The mechanism which is fundamentally important in the processis the in situ production of an acid addition petroleum sludge which isretained on the walls of the porous media of the formation and becomes afuel to sustain the forward in situ combustion process. The acidaddition petroleum sludge can be produced by an admixture of an acid andcrude oil. Combustion of these petroleum sludges produces an acid gas,which upon flowing to cooler regions of the reservoir is contacted withwater to establish a regenerative cycle of acid. Several techniques havebeen described for practically applying this method of contacting highgravity crude oil in the reservoir with one or more acids, acid gases,combustion products of certain fuel gases and/or combustion prodnets ofcrude oils which would naturally contain acid gases, especially oxidesof sulphur and nitrogen. The particular technique used will depend uponsuch factors as reservoir conditions, availability of the fuel involvedin each technique, and the type of crudeoil in the reservoir.

While particular embodiments of the present invention have been shownand described, it is apparent that changes and modifications may be madewithout departing from this invention in its broader aspects, andtherefore, the aim in the appended claims is to cover all such changesand modifications as fall within the true spirit and scope of thisinvention.

What is claimed is:

1. A method for the production of hydrocarbon materials from a formationpenetrated by at least two wells and containing high gravity petroleumhydrocarbon deposits which includes: injecting air and an acid throughone of the wells into the formation, which acid forms a sludge with atleast a portion of the hydrocarbon deposits in the formation; applyingheat from an external source to the sludge formed in the formation whilecontinuing the injection of air into the formation to initiatecombustion of the sludge; ceasing the application of heat from suchexternal source while continuing the injection of air to support aregenerative cycle of combustion of such sludged material; andrecovering hydrocarbons liberated by such combustion from the otherwell.

2. The method of claim 1 in which the amount of acid injected issufficient to generate a sludge that will support a regenerative cycleof radial combustion throughout the formation between the wells.

3. The method of claim 1 in which combustion of the sludge materialliberates an acid gas and water vapor which condenses when contactedwith cooler zones of the formation to form an acid which, in turn,reacts with the high gravity petroleum deposits to form a combustiblesludge and thereby produce a regenerative cycle of combustion.

4. A method of production'of hydrocarbon materials from a formationpenetrated by at least two wells and containing high gravity petroleumdeposits, which includes: injecting air through one of the wells intothe formation; while continuing the injection of air introducing a slugof inorganic acid into the one well for contacting the acid with suchhigh gravity petroleum products to form a petroleum acid sludge materialin the formation; introducing a temporary supply of heat to the sludgematerial while continuing the injection of air to ignite the sludgedmaterial; discontinuing the supply of heat while continuing theinjection of air to support a regenerative cycle of acid sludging andcombustion in the formation; and recovering hydrocarbon materialsliberated by the combustion from the other well.

5. A method for the production of hydrocarbon materials from a formationpenetrated by at least two wells and containing high gravity petroleumdeposits, which includes: applying heat to the formation through one ofthe Wells; injecting air and a fuel gas into the one well, which fuelgas contains a substantial amount of acid forming elements that willform an acid gas when reacted with air by combustion, such acid gasesforming an acid when combined with water in the well and formation toproduce a petroleum acid sludge material in the formation; discontinuingthe injection of fuel gas while continuing the injection of air tosupport a regenerative cycle of sludging and combustion of the sludgedmaterial; and producing hydrocarbon materials liberated by suchcombustion from the other well.

6. The method of claim 5, in which combustion of the sludged materialliberates an acid gas which condenses when contacted with cooler zonesof the formation and combines with water in the formation to form anacid which, in turn, reacts with the high gravity petroleum deposits toform a combustible sludge and thereby form a regenerative cycle.

7. A method for the production of hydrocarbon materials from a formationpenetrated by at least two wells and containing high gravity petroleumhydrocarbon deposits, which includes: injecting air and a slug of crudeoil through one of the wells into the formation, where the injectedcrude oil contains a relatively high percentage of compounds for formingan acid when reacted with air by combustion and water in the formation,such acid forming a combustible acid sludge material when contacted withhigh gravity petroleum hydrocarbon deposits; applying heat to the sludgematerial while continuing the injection of air to ignite and burn thesludge material, the combustion of the sludged material liberating anacid gas which condenses after flowing to cooler regions of theformation and combines with water to form a regenerative acid which, inturn, combines with new high gravity hydrocarbon deposits to form acombustible acid sludge and thereby sustain burning throughout theformation; and recovering hydrocarbons liberated by the 'burn ing fromthe other well.

References Cited UNITED STATES PATENTS 2,863,510 12/1958 Tadema et al.166-39 X 2,889,881 6/1959 Trantham et al. 16611 3,064,728 11/1962 Gould166-11 X 3,233,671 2/1966 Chatenever 16611 3,263,750 8/1966 Hardy 166113,400,760 9/ 1968 Orkiszewski 16611 STEPHEN J. NOVOSAD, PrimaryExaminer.

US. Cl. X.R.

